My friend has a 9 year old only son and handles this beautifully-I've learned a lot form her for my oldest daughter who is bored out of her mind unless someone is playing WITH her. My friend runs her own business from home, and her son used to be incapable of self play, and she had to resolve it.
First of all, clear your conscience, it is TOTALLY OK that you don't always want to play with him, and he's old enough to learn you're not always free. That said, you really don't ever need the words, "I don't want to do this with you", so his feelings won't be hurt.
Organize things for him first. Give him a few fun designated areas to play and things to do. Or just one, depending on your set up. In each area, keep only one or two activities-too many options tends to make kids check out. Simplicity is key.
Let him know you have work you need to do and he must play for (give specific window). Get him started, and then let him know you've got to go, point at the clock and let him know how long he needs to play. It may take some enforcing at first, but it's a skill he needs and a lesson that moms aren't always available to play.
If I'm painting in the studio, or working on business and my son is in his nap, I set up a clear table for my daughter (4) with play dough, a coloring book, a dress up doll, her stable and horses...just ONE THING at a time with all the accessories it needs and I let her know I cannot play with her until such and such a time. She used to keep wandering over to lean on me etc, but she got it when I treated it like a rule and never backed down.
Now she likes the security of the schedule, has learned that momma has important business to attend to, and tells her brother not to bug me for those increments of time, and she's learned to tell time really well! She also got better at being inventive. It took a few weeks. Good luck!